1. Field of the Invention
This invention relates to a method for decreasing the catalytic activity of metal contaminants on cracking catalyst. More specifically this invention is directed to a method for reducing the detrimental effects of metal contaminants, such as nickel, vanadium and/or iron, which have become deposited upon cracking catalyst from feedstock containing same.
2. Description of the Prior Art
In the catalytic cracking of hydrocarbon feedstocks, particularly heavy feedstocks, vanadium, nickel and/or iron present in the feedstock becomes deposited on the cracking catalyst promoting excessive hydrogen and coke makes. These metal contaminants are not removed during conventional catalyst regeneration operations during which coke deposits on the catalyst are converted to CO and CO.sub.2. As used hereinafter the term "passivation" is defined as a method for decreasing the detrimental catalytic effects of metal contaminants such as nickel, vanadium and iron which become deposited on catalyst.
U.S. Pat. Nos. 3,711,422; 4,025,545; 4,031,002; 4,111,845; 4,141,858; 4,148,712; 4,148,714 and 4,166,806 all are directed to the contacting of the cracking catalyst with antimony compounds to passivate the catalytic activity of the iron, nickel and vanadium contaminants deposited on the catalyst. However, antimony compounds, alone, may not passivate the metal contaminants to sufficiently low levels particularly where the metal contaminant concentration on the catalyst is relatively high. U.S. Pat. No. 4,176,084 is directed to the passivation of metals contaminated catalyst in a regeneration zone operated for incomplete combustion of the coke to CO.sub.2 by periodically increasing the oxygen concentration above that required for complete combustion of the coke and by maintaining the temperature above 1300.degree. F. This patent does not disclose a method for passivating metals-contaminated catalyst in a system where the regeneration zone is routinely operated for complete combustion of the coke.
U.S. Pat. No. 4,162,213 is directed at decreasing the catalytic activity of metal contaminants present in cracking catalyst by regenerating the catalyst at temperatures of 1300.degree.-1400.degree. F. in such a manner as to leave less than 0.10 wt. % residual carbon on the catalyst.
Cimbalo, Foster and Wachtel in an article entitled "Deposited Metals Poison FCC Catalyst" published at pp 112-122 of the May 15, 1972 issued of Oil and Gas Journal disclose that the catalytic activity of metal contaminants decreases with repeated oxidation and reduction cycles.
U.S. Pat. No. 3,718,553 is directed at the use of a cracking catalyst impregnated with 100-1000 parts per million by weight (WPPM) of iron, nickel or vanadium or a combination of these metals to increase the octane number of the cracked hydrocarbon products. This reference does not recognize that use of certain of these metals may adversely affect the catalyst selectivity or activity.
The subject invention is directed at the passivation of catalyst metal contaminants by passing the catalyst through a reduction zone maintained at an elevated temperature. The passivation process improves the selectivity and activity of the catalyst thereby permitting the amount of conventional passivation promoter to be reduced or eliminated.